The present invention relates to vacuum pumps and more particularly to screw pumps.
Screw pumps usually comprise two spaced parallel shafts each carrying externally threaded rotors, said shafts being mounted in a pump body such that the threads of the rotors intermesh. Close tolerances between the rotor threads at the points of intermeshing and with the internal surface of the pump body, which acts as a stator, causes volumes of gas being pumped between an inlet and an outlet to be trapped between the threads of the rotors and said internal surface and thereby urged through the pump as the rotors rotate.
Such screw pumps are potentially attractive since they can be manufactured with few working components and they have an ability to pump from a high vacuum environment at the inlet down to atmospheric pressure at the outlet.
Conventional screw pumps with shafts which are either mounted cantilever fashion within the pump body or supported at each end with bearings use a common head plate or plates to support the bearing or bearings of both shafts. The head plate or plates are then fixed to the pump body.
This construction has several disadvantages, for example, the head plate(s) has to be or is usually cooled to keep the bearing within its operating temperatures. However, the pump body (stator) is often run much hotter particularly on screw pumps used in semiconductor manufacturing processes. This gives rise to differential thermal expansion such that the stator bores move apart but the head plate and bearings and therefore the rotors do not move as far and become off-centered relative to their respective bores. This requires the screw pump to be made with large running clearances to prevent or minimize the possibility of seizure.
Further, accurate centering of the shafts and rotors within the bores is also difficult due to the tolerance stack-up of the bearing centers and the positioning of the head plate relative to the stator which is typically effected with doweling.
It is an aim of the present invention to provide a screw pump which prevents or mitigates against the problems associated with conventional screw pumps and in particular the problem of thermal distortion.